Transducer for fm sonar application

ABSTRACT

A narrow beam, electroacoustic sonar transducer is described that substantially reduces acoustic energy cross-coupling between projecting and receiving portions thereof during continuous F.M. use. The transducer is characterized by a pair of semi-annular piezoelectric projecting elements arranged in concentric relation to a pair of semi-circular, piezoelectric receiving elements all lying in a common plane. The reverse sides of the elements are slotted, and the spaces between the elements are filled with acoustic energy transfer barrier material. The piezoelectric elements and acoustic energy barrier material are encapsulated as an assembly in a layer of deaerated polyurethane material.

TRANSDUCER FOR FM SONAR APPLICATION Rufus L. Cook, Panama City, Fla.

The United States of America as represented by the Secretary of theNavy, Washington, DC.

Filed: Oct. 12, 1973 Appl. No.: 406,069

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 8/1955 Bradfield 310/96 2/1962Henry 310/96 X 3/1966 Roshon, Jr. et al.. 340/8 R X 10/1966 Parssinen340/10 X 6/1967 Dorr et a1. 310/96 X Primary ExaminerMark O. BuddAttorney, Agent, or FirmRichard S. Sciascia; Don D. Doty; Harvey A.David [5 7] ABSTRACT A narrow beam, electroacoustic sonar transducer isdescribed that substantially reduces acoustic energy cross-couplingbetween projecting and receiving portions thereof during continuous RM.use. The transducer is characterized by a pair of semi-annularpiezoelectric projecting elements arranged in concentric relation to apair of semi-circular, piezoelectric receiving elements all lying in acommon plane. The reverse sides of the elements are slotted, and thespaces between the elements are filled with acoustic energy transferbarrier material. The piezoelectric elements and acoustic energy barriermaterial are encapsulated as an assembly in a layer of deaeratedpolyurethane material.

11 Claims, 3 Drawing Figures TRANSDUCER FOR FM SONAR APPLICATIONSTATEMENT OF GOVERNMENT INTEREST The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for Governmental purposes without the payment of any royaltiesthereon or therefor.

FIELD OF THE INVENTION This invention relates to the conversion ofenergy between electrical and acoustical states, and more particularlyto narrow beam, wide band, electroacoustic transducers capable ofsimultaneously generating and receiving acoustical energy, such as in acontinuous F.M. (frequency modulated) sonar system. In this regard theinvention is directed especially to the problems of reduction of what issometimes known as cross-talk between the electroacoustic elements. Forpurposes of this specification, the term crosscoupling" will be used asbeing more descriptive of the detrimental condition that is sought to bereduced. Thus, cross-coupling will be considered to be the transfer ofacoustical energy from a projecting element to a receiving elementthrough a path or paths other than the primarily intended projection andreception paths to and from a reflecting target. Cross-couplingtherefore includes mechanical transfer of energy through the elementsupporting structures, through the elements themselves, and/or throughthe surrounding environment.

DISCUSSION OF THE PRIOR ART Various narrow beam electroacoustictransducer devices have been proposed that utilize separate projectingand receiving elements in a unitary structure, often with the elementsof circular or annular configuration and mounted concentrically in aneffort to achieve efficient narrow beam transmission and reception.Examples of such devices are found in US. Pat. Nos.

3,l09,1l2 of R. A. Lester, 3,327,286 of J. A. Dorr et al, and 3,457,543of O. I... Akervold et al. None of the known narrow beam projecting andreceiving transducers, however, are satisfactory for use in a continuousF .M. sonar system because of signal degradation due to cross-coupling.Of course, the problem of crosscoupling or cross-talk is minimized oravoided when such transducers are used in pairs to transmit and receivetwo widely different frequencies, or where the transducers are used inpulsed C.W. (carrier wave) systems such as in depth sounders or waveheight measuring devices wherein the projecting and receiving duties ofthe transducer are alternated. Accordingly, most of the prior art effortin narrow beam electroacoustic projecting and receiving transducers hasbeen toward improving resolution, as by suppression of side lobes andenhancement of main lobes. This has left a gap or need in theelectroacoustic transducer art for a narrow beam electroacousticprojecting and receiving transducer that is capable of simultaneouslyand continuously v transmitting and receiving F.M. acoustic energy intoand from a water medium with a minimum of signal degradation or loss ofoperating efficiency due to crosscoupling.

SUMMARY OF THE INVENTION The invention aims to overcome most or all ofthe disadvantages of the prior art in filling the need for a narrowbeam, wide band, projecting and receiving electroacoustic transducer,that is notably superior for use in continuous F.M. sonar systems,through the reduction of cross-coupling, and the optimization of energytransfer through the desired paths.

Accordingly, it is a general object of the invention to provide animproved projecting and receiving electroacoustic transducer.

A more specific object of the invention is the provision of anelectroacoustic transducer for projection and reception of acousticenergy by structurally associated piezoelectric projecting and receivingelements, respectively, characterized by a minimum of energycross-coupling therebetween, whereby the transducer is more useful forcontinuous F.M. service in high KHZ ranges than transducers availableheretofore.

Another, and important, object of the invention is the provision of anovel transducer of the foregoing character that is compact and ruggedin construction making it suitable for use under extreme conditions andreliable for use either alone or in large arrays.

Still another object is to provide an electroacoustic transducer for,but not limited to, continuous F.M. sonar use that exhibitssubstantially uniform energy transfer across a substantial frequencybandwidth.

Yet another object is the provision of a transducer structure having thementioned features and which is characterized by a pair of semi-annularor arcuate piezoelectric transducer elements lying in a plane normal tothe-direction of transmission, the ends of the arcuate elements beingseparated by a vibrational energy transfer barrier material, and a pairof semi-circular piezoelectric elements, disposed in the same plane andin concentric relation to the arcuate pair of elements, thesemi-circular elements being separated from one another and from thefirst pair of elements by vibrational energy transfer barrier material.

A further object of the invention is to enhance the frequency bandwidthcharacteristics, as well as reducing cross-coupling, of the transducersmade according to the invention by dicing or slotting the rear surfacesof the pairs of piezoelectric elements, and backing the elements withenergy transfer barrier material.

As another object the invention aims to provide an improved projectingand receiving transducer of continuous F.M. use that is characterized bya particularly low degree of cross-coupling between the projecting andreceiving elements on the order of 35 db or less.

Other objects and many of the attendant advantages will be readilyappreciated as the subject invention becomes better understood byreference to the following detailed description, when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THEDRAWINGS FIG. 1 is a front elevational view of atransducer embodying the invention with portions broken away to revealother portions;

FIG. 2 is a sectional view of the transducer taken substantially alongline 2-2 of FIG. 1; and

FIG. 3 is a rear elevational view of one of the piezoelectric elementsof the transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the form of the inventionillustrated in the drawings and described hereinafter, there isdescribed an electroacoustic transducer, indicated generally at 10, thatis particularly well suited for use in simultaneously projecting andreceiving acoustic energy into and from an aqueous medium as part of ahand held, continuous duty, F.M. sonarsystem operating in a frequencyrange of 100 to 150 KHz, and operable as a hydrophone in a lower 24 to45 KHz range. Such F.M. sonar systems are well known to those skilled inthe art to which the invention pertains and, accordingly, descriptionthereof is not deemed pertinent here. Suffice it to say, for the purposeof this discussion, that such systems operate at frequencies that maydiffer considerably from what are generally considered audible, and thatthe terms acoustic, electroacoustic, or the like are not to be limitedto audible frequency ranges.

Transducer comprises, among its salient features, two pairs ofpiezoelectric elements, a first pair consisting of semi-circular or halfdisc elements 12 and 14, and a second pair consisting of semi-annularelements 16 and 18. The first pair of piezoelectric elements 12 and 14are arranged with their straight edge surfaces 12a and 14a in spacedparallel relation, and their curved, substantially semi-circular edgesurfaces 12b and 1412, respectively, facing outwardly and lying on acircle.

The second pair of piezoelectric elements 16 and 18 are disposed withtheir inner curved edge surfaces 16a and 18a in concentric, spacedrelation to edge surfaces 12b and 14b, respectively, of elements 12 and14. The straight end surfaces 16b and 18b of semi-annular piezoelectricelements 16 and 18 are respectively spaced from one another with thespaces therebetween being aligned with the space between straight edges12a and 14a of semi-disc piezoelectric elements 12 and 14. Thesepiezoelectric elements 12, 14, 16, and 18 are conveniently formed bycutting a solid disc of ceramic piezoelectric material, such as bariumtitanate or the like, that has been polarized for thickness mode ofoperation. Thus, as shown in FIG. 3, an original disc, having a diameterequal to the desired diameter of the pair of elements 16 and 18, is cut,for example by a well known air blast and abrasive technique, into foursegments having the desired configurations. The disc from which theelements are cut preferably has been provided with thin layers ofelectrically conductive electrode material on the opposite flat facesthereof in a manner well understood in the art. These front and rearelectrode layers are illustrated respectively at 22 and 24 in FIG. 2.

Piezoelectric elements 12, 14, 16, and 18 have plane radiating andreceiving surfaces on their forward, or aqueous medium facing sides,whereas the rear sides of those piezoelectric elements are diced orslotted in a rectilinear pattern, best illustrated in FIG. 3, by aplurality of intersecting slots 30. These slots are conveniently formedby means of a high speed, diamond blade saw to a depth of approximately85 percent of the thickness of the piezoelectric elements. In theembodiment being described, slots 30 were positioned to leave squarelands or posts 32 in rows and columns having center-to-center spacingsof 0.1 inch. In the present example, slot widths of 0.0 l 5 inch wereused, and spacings between the piezoelectric elements 12, 14, 16, and 18of 0.065 inch. Further, in that embodiment, which was designed to beoperated in a continuous transmission F.M. system in the 100 KHz to I50KHz frequency range, the diameter across elements 16 and 18 was about3.50 inches, and the diameter across elements 12 and 14 was 0.68 of thediameter across elements 16 and 18, or about 2.38 inches. This ratio ofthe outer elements to the inner elements is important to efficientoperation of transducer devices embodying the invention.

The rearwardly directed faces of the piezoelectric element posts formedby the intersecting slots 30 are provided with an electrode 36 in theform of a metal mesh or grid soldered to the free end of each post. Thisrear electrode may be formed of copper, nickel, or any other convenientmetal or metal alloy. In the embodiment being described, electrode 36comprises a mesh of 0.005 inch copper wire.

Suitable lead wires, such as 38 and 40, are connected to the frontelectrodes 22, while one or more other lead wires, such as 42, areconnected to rear electrode 36.

The spaces between the respective piezoelectric elements l2, 14, 16 and18 are filled with an acoustic energy barrier material 46. Material 46is preferably one having good sound insulation or acoustic energyabsorption characteristics, one example being the mate rial sold underthe trademark CORPRENE. This material is readily available in sheetform. In the present embodiment the spaces 13, 17 and 19 were filledwith material 46 cut from sheet stock having a thickness of l/16 inchand cemented in place by a commercial adhesive or cement sold under thetrademark VUL- CALOCK. Layers of the acoustic energy barrier ma terial46 are also applied by cementing a strip of COR- PRENE to the peripheryof the assembled semiannular piezoelectric elements 16 and 18, and bycementing a disc of CORPRENE to the rear faces of the assembled elements12, 14, 16, and 18 over the wire mesh rear electrode 36.

Only the transmitting and receiving front surfaces of the piezoelectricelements remain free of covering with the material 46, these being thesurfaces which are to be acoustically coupled to a surrounding aqueousmedium when in use. To this end, these front faces of the piezoelectricelements are provided with an acoustic window comprising a layer of asubstantially acoustically transparent material. This is accomplished,along with making the aforedescribed construction an integral andwaterproof unit, by encapsulating or coating with a suitableacoustically transparent material 50.

The material 50 from which acoustic window layer and encapsulation isperformed is preferably one that exhibits a relatively high and uniformdegree of energy transmission, and a relatively low degree of energyreflectance, throughout the range of frequencies in which the transducer10 will be operated. Such a material is exemplified by a product sold asa polyurethane potting compound under the trademark HYSOL.

In the present embodiment, the encapsulating layer 50 of polyurethaneacoustic window material is formed by mixing the two components suppliedand then deaerating the mixture before casting around the assembly ofelements 12, 14, 16 and 18, the electrode mesh 36, and the acousticenergy absorbing barrier material 46. The resulting polyurethane layer50, has a nominal thickness of 0.060 inches on the radiating andreceiving surfaces of the piezoelectric elements.

The transducer assembly as thus far described is bonded by a suitableadhesive to the end wall 55a of an aluminum housing 55 for transmitterand receiver electronics of the sonar system with which transducer isintended to be used. A resiliently flexible boot 58 of an acousticallytransparent, waterproof material such as Rho-C rubber is applied overthe transducer assembly, and secured to housing 55 as by an encirclingband 60. Application of boot 58 is facilitated if it is first lubricatedwith mineral oil.

Transducer 10 is characterized in its operation by a particularly lowcross-coupling level of 35 db, or lower, between the projecting elements12, 14 and the receiving elements 16, 18. The low level of crosscouplingis attributable in part to suppression of acoustic radiation, orcoupling, in undesired directions. Contributin g to this is the use ofCORPRENE or similar isolation material 46 between and around, and behindthe piezoelectric elements, the slotting or dicing of the rear faces ofthe piezoelectric elements to create voids between the post portions ofthe elements, and the interposition of the wire mesh electrode 36between the rear faces of the piezoelectric elements and the layer ofCORPRENE material therebehind. A further important factor results fromthe transmitting piezoelectric elements being in the form of a pair ofhalf-discs instead of a single complete disc, and from the receivingelements being semi-annular rather than a complete annulus.

Obviously, other embodiments and modifications of the subject inventionwill readily come to the mind of one skilled in the art having thebenefit of the teachings presented in the foregoing description and thedrawings. It is, therefore, to be understood that this invention is notto be limited thereto and that said modifications and embodiments areintended to be included within the scope of the appended claims.

What is claimed is: 1. An electroacoustic transducer comprising: firstand second pairs of piezoelectric elements; said piezoelectric elementsof said first pair being substantially semi-disc shaped and presentingflat front surfaces each bounded by a straight side surface and anarcuate side surface, said piezoelectric elements of said first pairbeing disposed with said front surfaces in a first common plane withsaid straight side surfaces in adjacent, spaced, parallel relation toone another and said arcuate surfaces lying in a circle; saidpiezoelectric elements of said second pair being substantiallysemi-annular in shape and presenting flat front surfaces each bounded byinner and outer arcuate surfaces and straight end surfaces, saidpiezoelectric elements of said pair being disposed in concentricrelation to said first pair with said front surfaces lying in said firstcommon plane and said inner arcuate surfaces in uniformly spacedrelation to said arcuate surfaces of said piezoelectric elements of saidfirst pair, said straight end surfaces being in spaced parallel relationto one another;

said piezoelectric elements of said first and second pairs being furthercharacterized by flat rear surfaces lying in a second common plane;

front electrode means, on each of said front surfaces,

for effecting electrical connection therewith;

rear electrode means, on said rear surfaces, for effecting electricalconnection therewith; and

said piezoelectric elements each being further characterized by aplurality of intersecting slots formed in said rear surfaces anddefining a plurality of rearwardly extending posts on each of saidpiezoelectric elements. 2. An electroacoustic transducer as defined inclaim 1, and wherein:

said rear electrode means comprises a metal mesh soldered to said rearsurfaces at the end of each of said posts. 3. An electroacoustictransducer as defined in claim 2, and further comprising:

acoustic energy barrier material disposed between said straight sidesurfaces of said piezoelectric elements of said first pair, between saidend surfaces of said piezoelectric elements of said second pair, andconcentrically between said first and second pairs of piezoelectricelements. 4. An electroacoustic transducer as defined in claim 3, andfurther comprising:

a layer of acoustic barrier material disposed in encircling engagementwith said outer arcuate surfaces of said piezoelectric elements of saidsecond pair.

5. An electroacoustic transducer as defined in claim 4, and furthercomprising:

a disc-shaped layer of said acoustic energy barrier means disposedagainst said rear electrode means in covering relation to said first andsecond pairs of piezoelectric elements.

6. An electroacoustic transducer as defined in claim 5, and wherein:

said spaced end surfaces of said second pair of piezoelectric elementsare aligned with said straight side surfaces of said first pair ofpiezoelectric elements.

7. An electroacoustic transducer as defined in claim 6, and furthercomprising:

electrically insulating and waterproof acoustic window means, overlyingsaid front surfaces of said first and second pairs of piezoelectricelements and said acoustic barrier material therebetween, for effectingacoustic coupling between said front surfaces and an ambient watermedium.

8. An electroacoustic transducer as defined in claim 7, and wherein:

said acoustic window means comprises a layer of substantiallyacoustically transparent deaerated polyurethane material.

9. An electroacoustic transducer as defined in claim 8, and wherein:

said acoustic window means further comprises a layer of substantiallyacoustically transparent rubber-like material.

10. An electroacoustic transducer for simultaneous projection andreception of acoustic energy into and from an aqueous medium withminimized crosscoupling, said transducer comprising:

a pair of substantially semi-disc shaped, piezoelectric projectingelements having arcuate side surfaces arranged on a first circle andstraight side surfaces in parallel, spaced relation, said projectingelements being polarized for thickness excitation between front and rearsurfaces thereof;

a pair of substantially semi-annular shaped, piezoelectric receivingelements having inner and outer arcuate side surfaces respectivelyarranged on second and third circles concentric with said first circleand straight end surfaces disposed in spaced parallel relation to oneanother, said receiving elements being polarized for thicknessexcitation between front and rear surfaces thereof;

acoustic energy absorbing material disposed between said straight sidesurfaces of said projecting elements, between said straight end surfacesof said receiving elements, and between said arcuate side surfaces ofsaid projecting elements and said inner arcuate side surfaces of saidreceiving elements;

said projecting and receiving elements being characterized byintersecting grooves formed in said rear surfaces and extending througha majority of the thickness of said elements so as to define a pluralityof rearwardly extending, discrete posts and so as to interrupt planarvibration transmission within each of said elements;

front electrode means comprising a layer of electrically conductivematerial on said front surfaces of each of said piezoelectric elements;

rear electrode means comprising electrically conductive wire meshoverlying said rear surfaces and electrically bonded thereto, said wiremesh being loosely woven so as to be substantially free of vibratoryenergy transmission from point to point thereof;

a layer of acoustic energy absorbing material disposed behind said wiremesh in loosely coupled engagement therewith;

a layer of acoustic energy absorbing material in encircling relation tosaid first and second piezoelectric elements;

a layer of substantially acoustically clear deaerated 10, and wherein:

said straight side surfaces of said projecting elements are in alignmentwith said straight end surfaces of said receiving elements.

1. An electroacoustic transducer comprising: first and second pairs ofpiezoelectric elements; said piezoelectric elements of said first pairbeing substantially semi-disc shaped and presenting flat front surfaceseach bounded by a straight side surface and an arcuate side surface,said piezoelectric elements of said first pair being disposed with saidfront surfaces in a first common plane with said straight side surfacesin adjacent, spaced, parallel relation to one another and said arcuatesurfaces lying in a circle; said piezoelectric elements of said secondpair being substantially semi-annular in shape and presenting flat frontsurfaces each bounded by inner and outer arcuate surfaces and straightend surfaces, said piezoelectric elements of said pair being disposed inconcentric relation to said first pair with said front surfaces lying insaid first common plane and said inner arcuate surfaces in uniformlyspaced relation to said arcuate surfaces of said piezoelectric elementsof said first pair, said straight end surfaces being in spaced parallelrelation to one another; said piezoelectric elements of said first andsecond pairs being further characterized by flat rear surfaces lying ina second common plane; front electrode means, on each of said frontsurfaces, for effecting electrical connection therewith; rear electrodemeans, on said rear surfaces, for effecting electrical connectiontherewith; and said piezoelectric elements each being furthercharacterized by a plurality of intersecting slots formed in said rearsurfaces and defining a plurality of rearwardly extending posts on eachof said piezoelectric elements.
 2. An electroacoustic transducer asdefined in claim 1, and wherein: said rear electrode means comprises ametal mesh soldered to said rear surfaces at the end of each of saidposts.
 3. An electroacoustic transducer as defined in claim 2, andfurther comprising: acoustic energy barrier material disposed betweensaid straight side surfaces of said piezoelectric elements of said firstpair, between said end surfaces of said piezoelectric elements of saidsecond pair, and concentrically between said first and second pairs ofpiezoelectric elements.
 4. An electroacoustic transducer as defined inclaim 3, and further comprising: a layer of acoustic barrier materialdisposed in encircling engagement with said outer arcuate surfaces ofsaid piezoelectric elements of said second pair.
 5. An electroacoustictransducer as defined in claim 4, and further comprising: a disc-shapedlayer of said acoustic energy barrier means disposed against said rearelectrode means in covering relation to said first and second pairs ofpiezoelectric elements.
 6. An electroacoustic transducer as defined inclaim 5, and wherein: said spaced end surfaces of said second pair ofpiezoelectric elements are aligned with said straight side surfaces ofsaid first pair of piezoelectric elements.
 7. An electroacoustictransducer aS defined in claim 6, and further comprising: electricallyinsulating and waterproof acoustic window means, overlying said frontsurfaces of said first and second pairs of piezoelectric elements andsaid acoustic barrier material therebetween, for effecting acousticcoupling between said front surfaces and an ambient water medium.
 8. Anelectroacoustic transducer as defined in claim 7, and wherein: saidacoustic window means comprises a layer of substantially acousticallytransparent deaerated polyurethane material.
 9. An electroacoustictransducer as defined in claim 8, and wherein: said acoustic windowmeans further comprises a layer of substantially acousticallytransparent rubber-like material.
 10. An electroacoustic transducer forsimultaneous projection and reception of acoustic energy into and froman aqueous medium with minimized cross-coupling, said transducercomprising: a pair of substantially semi-disc shaped, piezoelectricprojecting elements having arcuate side surfaces arranged on a firstcircle and straight side surfaces in parallel, spaced relation, saidprojecting elements being polarized for thickness excitation betweenfront and rear surfaces thereof; a pair of substantially semi-annularshaped, piezoelectric receiving elements having inner and outer arcuateside surfaces respectively arranged on second and third circlesconcentric with said first circle and straight end surfaces disposed inspaced parallel relation to one another, said receiving elements beingpolarized for thickness excitation between front and rear surfacesthereof; acoustic energy absorbing material disposed between saidstraight side surfaces of said projecting elements, between saidstraight end surfaces of said receiving elements, and between saidarcuate side surfaces of said projecting elements and said inner arcuateside surfaces of said receiving elements; said projecting and receivingelements being characterized by intersecting grooves formed in said rearsurfaces and extending through a majority of the thickness of saidelements so as to define a plurality of rearwardly extending, discreteposts and so as to interrupt planar vibration transmission within eachof said elements; front electrode means comprising a layer ofelectrically conductive material on said front surfaces of each of saidpiezoelectric elements; rear electrode means comprising electricallyconductive wire mesh overlying said rear surfaces and electricallybonded thereto, said wire mesh being loosely woven so as to besubstantially free of vibratory energy transmission from point to pointthereof; a layer of acoustic energy absorbing material disposed behindsaid wire mesh in loosely coupled engagement therewith; a layer ofacoustic energy absorbing material in encircling relation to said firstand second piezoelectric elements; a layer of substantially acousticallyclear deaerated polyurethane compound encapsulating said projecting andreceiving elements, said electrode means, and said acoustic energyabsorbing material; and conductor means, connected to said electrodemeans, for applying energizing electrical signals to said projectingelements and for collecting electrical signals from said receivingelements.
 11. An electroacoustic transducer as defined in claim 10, andwherein: said straight side surfaces of said projecting elements are inalignment with said straight end surfaces of said receiving elements.